1. Field of the Invention
The present invention is referred to a monolithic float glass forming chamber and more specifically to a monolithic glass forming chamber and a method for the construction of the same using castable and pumpable refractory to form monolithic refractory structures by sol-gel process on site of construction.
2. Description of the Related Art
The recently developed sol-gel refractory technology, have had its mayor and more widely application in the field of metallurgical processes, like repairing and constructing linings for molten metal containers and incinerators kilns.
In a research made on related art in present subject, has shown that the application of such sol-gel refractory technology to the glass manufacturing processes is relatively limited and mainly focused towards the repair of some wear areas in the refractory structures of the glass melting furnaces. Even more specifically, none reference was found related with the application of such sol-gel refractory technology for the construction of the refractories for a float glass forming chamber.
In order to have a better understanding of the importance of the method for the construction of the refractories structures for the whole glass forming chamber, the following is an explanation about the development of knowledge and skills that have been applied trough the time in this part of the process for the glass manufacture.
Since the beginning of the application of the float process for the flat glass production about 50 years ago, the improvement in the knowledge and skills in different concurrent specialties, have resulted in very important advances incorporated in systems, materials and methods in order to improve the process control and the quality of the glass produced, besides increasing the length of campaigns of the production units having now a day targets for 14 or more years of continuous operation.
In the case of the refractory materials that form part as elements for the operation or compose within a float bath structure in the float process for manufacturing flat glass better compositions and better qualities have been developed to support the particular conditions that prevail inside the forming chamber and with glass contact positions. Also to meet such requirements and overcome or solve different type of problems aroused through the time in refractory materials located in each section of the float bath. For example for the refractories used in canal, lip, tweel and bottom blocking, different methods to manufacture had been developed and the same tendency has being observed for the roof tiles refractories assembly. Some examples are included in the next references in order to explain the diversity and complexity of such changes and developments incorporated in refractory materials for the float glass process.
At the hot end of the float process, were the molten glass coming from the melting process is continuously poured trough the canal and lip into the float bath as it is explained in U.S. Pat. No. 3,220,816, showing a method for delivering molten glass on the molten tin in a controlled way and falling freely from the spout lip to form a steady shape of glass flowing from the point of contact of such glass with the surface of molten tin the glass thereon by spreading laterally and flowing both backward and forward.
To control the flow rate of glass entering the float bath have been used a refractory element called tweel located in the canal and before the lip, as is illustrated in U.S. Pat. No. 3,445,217, which states the use of a tweel formed by two different refractories, one of them of a selected as glass wear-resistant refractory forming the lower part in the glass contact section such as fused cast zirconia, alumina or corundum material, and for the upper portion of the tweel formed by a refractory material selected for its mechanical strength and also for its resistance to thermal shock such as the use of pre-fired fire clay bonded calcined kyanite or sillimanite.
The U.S. Patent No. 3,508,902 states the use of a refractory shape named as a wetback tile which is contacted by the rearward flow of the delivered flow of molten glass. It is stated by this method that the main function of such wet back tile is to divide the backward flow and direct the same backward flow of molten glass outward each side so that it each flow may join the forward flowing portion of the glass, helping to maintain a controlled and steady flow and conducting certain kind of faults like bubbles and inclusions coming from the area of the canal lip towards the edges were do not affect the saleable section of glass ribbon.
In U.S. Pat. No. 4,099,950 it is stated the use of fused silica as refractory material to conform both the tweel and the wet back tile. The use of such material is considered an improvement in order to avoid the presence any fine seeds attributable to glass-refractory contact with tweel and wet back tile also prevents possible presence of reams or non homogeneities fine lines with different composition to that of the forming glass and coming from the wear of the tweel.
In the case of refractories that constitute the roof of the float forming chamber, also several improvements have been incorporated through the time for example, in U.S. Pat. No. 4,311,508 using a castable refractory material and an arrangement of anchors to support the roof refractory from the top in the outside and to give a nearly flat and horizontal surface in the interior of the roof. In the process for construction of such structure are used forms made of metal or wood to support the castable refractory while this hardens. Such refractory roof structure considered to be nearly monolithic due to the fact that contains small length of joints helps to prevent formation of condensates from the atmosphere of the bath, reducing with this, risks of drippings and also consequently reduce glass production losses by this cause. Besides it is considered with this arrangement to reduce the formation of cracks compared with the conventional float roof design formed by interlocked refractory tiles. As it can be seen this design still requires the use of an arrangement of anchors to support the roof structure from the outside.
Another important invention related with design and method for construct the bath roof refractory is explained in U.S. Pat. No. 4,340,412. It is stated the advantage of use a simplified roof structure having a reduced number of vertically extending joints and openings formed by relatively large refractory tiles made by precasting refractory cement material giving pieces with a flat and horizontal surface in the interior and suspended from the top by an external metallic hanger arrangement. Such reduction in joint helps to reduce the formation and dripping of condensed volatiles which contaminates the glass ribbon and tend to cause production losses. This in conjunction with the use of horizontal electrical heating elements adapted from the side walls of the chamber so that no openings through the roof structure need be provided for heating elements.
In the case of the refractory bottom blocks which are a used as a common practice in the float glass process. Such blocks made of fireclay pre-fired in dimension generally 150 to 300 cm in thickness and varying dimensions up to 50 to 70 cm in the side in preferably in rectangular shapes several changes and improvements in methods, material compositions and properties have been incorporated trough the time in order to overcome or solve different type of problems.
In U.S. Pat. No. 4,233,047, it is stated a procedure for hot and in situ repair of delaminated float bath bottom refractory blocks using blocks as inserts of high alumina with similar shape to that of the delaminated section of clay block and containing such high alumina repair insert block interconnected drills filled with tungsten rods giving to such repair insert block a greater density than the density of tin. This repair insert block will sink and may be placed into the hole formed by the lost delaminated block. This procedure for hot and in situ repair can avoid mayor costs involved in the case to stop the process for a cold repair. This procedure helps to prevent risks of glass faults as bottom surface bubbles, risks of tin leakage due to tin attack to the metal casing and other benefits as disclosed in same patent.
Another example of problem related with the use of blocks and consequently extensive presence of joints in the bottom refractory structure is explained in U.S. Pat. No. 4,036,626 in which teaches a method for preventing tin leaks in a float bath using metallic seals. Such metallic seal made of thin sheets of a metal which can be located in the lower part of bottom blocks joints and such metal sheet when in contact with molten tin, when tin penetrates between block joints can form an alloy with higher density and higher melting point than tin and by this way can seal any point of penetration of tin through joints of bottom blocks
And related with procedures to manufacture or to build the refractory structures required for the float bath process in the forming chamber, one important factor that determines the design and methods up to now used for such purpose is the fact that refractories require to have their properties prior to their installation in the structure, specially for the refractory bottom blocks and for the refractory bath roof, the shapes require to have a great accuracy in properties and dimensions in order to avoid problems in their behavior during operation at working conditions. As it is established in the method of construction for the float glass forming chamber of the present application, the main properties and design requirements for such refractory structures can be obtained and improved by casting the same refractory structures on site and making use of a relatively new toll applied for the production of refractory materials called sol-gel refractory technology. We have found that the refractory materials obtained by this new technology provide several advantages for the construction of float glass forming chambers and eliminating several problems found in the past. With the method of the present application it is possible to construct refractory structures for the float glass forming chamber using the casting technique and the new sol-gel refractory technology eventually in three parts; that is, the bath bottom refractory section, the side wall refractory section and the bath roof refractory section.
The bath bottom refractory section and the side wall refractory sections are made by casting the refractory composition to form a monolithic structure. This allows having the refractory structure with eventually cero expansion joints exposed to molten tin on which the glass floats to be formed.
In the case of the bath roof refractory section, the method of construction, also allows to have a monolithic refractory structure with flexible access for maintenance of electric heating elements necessary to have a better control for the glass forming process.
The development of the sol-gel refractory technology can be derived for example with U.S. Pat. No. 5,900,382. In such patent it is stated the use of aqueous silica sol to form a binder in conjunction with phosphate and magnesia as accelerator for the process to gel. Such refractory binder found to be very useful to obtain refractories materials with wide compositions like alumina, zirconia, mullite and also alumina silicates that can harden and have good properties after short time of drying at room temperature and without requirement of firing as it is the common practice for mayor types of refractory materials.
The sol-gel refractory technology has had mayor application in the metallurgical processes for the production of iron and steel. In these processes the sol-gel refractory technology is applied for the repair and construction of refractories linings as it is stated in U.S. Pat. No. 5,632,937. In such patent it is explained a method for applying a refractory lining directly inside a metallic vessel with reduced access for the installation of molds prefabricated and instead of this, such forms or molds are assembled inside the metallic vessel and the refractory lining is formed using a castable refractory compositions prepared also in situ. Such castable refractory compositions are cast between the forms and the metallic vessel giving the required refractory lining after allowing hardening. Such method to apply the refractory linings has been proved to reduce the costs and time of such operation and can be applicable also to other kind waste incinerators and rotary kilns.
More recently, some applications of the sol-gel refractory technology for the glass manufacture process have been done as it is stated by in U.S. Pat. No. 7,176,153. Such patent describes the method for repair a glass melting furnace using colloidal silica refractories and containing mixtures with alumina, zirconia and silica compositions with a silica binder. This method for the application of such colloidal silica refractories is by means of casting, pumping, or shotcreting and mainly directed for the repair of wear parts of glass melting furnaces whether using cast blocks previously prepared or applying directly onto the wear section of the refractory structure and mainly located in contact with molten glass, that is the bottom and side wall refractories of the glass melting furnace. Such invention, states that similar repairs can be done on other parts of the glass melting furnace apart from the bottom and side wall refractories and using same colloidal silica refractories containing mixtures of alumina, zirconia and silica compositions with a silica binder.
As it was stated, in the search carried out on the related art, no applications of the sol-gel refractory technology have been found to the construction of the refractories used in the forming chamber for the float glass manufacturing processes.